Field of the Invention
The present invention relates to a contactless power transmission device and a power transfer system, and particularly to a power control technique in a contactless power transmission device that transmits electric power to a power reception device in a contactless manner.
Description of the Background Art
Japanese Patent Laying-Open No. 2014-207795 discloses a contactless power feeding system that supplies electric power from a power feeding device (power transmission device) to a vehicle (power reception device) in a contactless manner. In this contactless power feeding system, the power feeding device includes a power transmission coil, an inverter and a control unit. The power transmission coil transmits electric power to the power reception coil mounted on the vehicle in a contactless manner. The inverter produces an AC current in accordance with a drive frequency for output to the power transmission coil. The control unit obtains a charging power command for a battery and output power for the battery from the vehicle side, and controls by feedback the drive frequency of the inverter such that the output power follows the charging power command.
When the inverter is a voltage-source inverter and supplies transmission power in accordance with the drive frequency to the power transmission unit, transmission power can be controlled by adjusting the duty of an inverter output voltage. By adjusting the drive frequency of the inverter, a turn-on current indicating an inverter output current at the rising of the inverter output voltage can be controlled.
For a power transmission device, a limit value can be placed on transmission power so as not to exceed a target value significantly. When such a limit value is placed, the transmission power needs to be maintained at a value less than or equal to the limit value.
When the drive frequency of the inverter is adjusted for controlling a turn-on current, the transmission power changes with the change in the drive frequency. When the transmission power changes with the adjustment of the drive frequency, the transmission power may exceed a limit value unexpectedly. For example, in the situation where the coupling coefficient between a power transmission coil and a power reception coil is small and the transmission power is small, changes in the transmission power tend to be large with respect to changes in the drive frequency. When the drive frequency of the inverter is adjusted, an overshoot of the transmission power may occur, so that the transmission power may exceed the limit value unexpectedly. Japanese Patent Laying-Open No. 2014-207795 mentioned above does not particularly discuss such a problem in that the transmission power exceeds a limit value unexpectedly and a solution thereof.